Solar cells are photovoltaic components for direct generation of electrical current from sunlight. Due to the growing demand for clean sources of energy, the manufacture of solar cells has expanded dramatically in recent years and continues to expand. Various types of solar cells exist and continue to be developed. Solar cells include absorber layers that absorb the sunlight that is converted into electrical current. The quality and performance of the absorber layer are therefore of paramount importance. The composition of the absorber layer and the structure of the absorber layer are of critical importance in order to maximize the efficiency of electrical current production. The formation of the absorber layer and its placement on the solar cell substrate are therefore also critical operations.
One particularly popular type of absorber material is a CIGS-based absorber material. CIGS—copper indium gallium selenide, Cu(In,Ga)Se2—is a popular chalcogenide semiconductor material and CIGS-based materials find use in various applications but are particularly useful as absorber layers in solar cells. In order to create a CIGS-based material, an indium, In, material must be formed on the solar cell substrate. The formation of the indium material is most commonly and most often accomplished by sputtering indium from an indium sputtering target onto the substrate. Other processing operations are of course used to form the other materials of the CIGS-based absorber layer. A shortcoming in the formation of the indium layer is that indium metal layers typically have large grains that are separated from each other resulting in an undesirably rough surface morphology. This is often thought to be due to the high-surface tension and low-melting temperature of indium due to its poor wettability. The undesirably rough surface morphology includes hillocks and reduces the efficiency of the absorber layer in producing electrical current from the photons in sunlight.
Various different sputtering conditions have been attempted to improve the surface morphology, i.e. reduce the surface roughness, of the indium layer. These previous attempts were not successful in improving the surface morphology and typically were time consuming and resulted in lower throughput.